A flexible circuit typically includes a substrate having one or more electrical conductors, circuits and/or printed circuit boards bonded thereto. The substrate is typically a roll of thick polymer film adapted for processing on reel-to-reel equipment. The reel-to-reel production method provides a cost effective manufacturing means.
To protect the electrical conductors from corrosion, shorting and other environmental factors, a first cover layer is adhered to the surface of the substrate (e.g., top surface) and the one or more electrical conductors. The first cover layer typically has a shrinkage coefficient that is greater than the substrate. Thus, upon cooling after the first cover layer is cured, the first cover layer curls the substrate/electrical conductor/cover layer sandwich structure. Hence, a second cover layer is, typically, adhered to the other surface of the substrate (e.g., bottom surface). The second cover layer provides an offsetting shrinkage coefficient such that the cover layer/substrate/electrical conductor/cover layer sandwich structure remains substantially flat.
The edges of the cover layer, which are in a state of tension due to the difference in shrinkage coefficients of the cover layers and the substrate, are stress concentrators. The stressed state of the cover layers can result in de-bonding during repetitive flexing of the flexible circuit. If the cover layers separate from the substrate, the electrical conductors become exposed subjecting them to corrosion, electrical shorting and the like. Furthermore, debonding may not directly expose the encapsulated part of the flexible circuit. Instead, debonding may deleteriously create a bubble or pocket which can act as a collection site for condensed water or solvent that diffuses through the encapsulating cover layer and can cause the exposure problems mentioned above. Additionally, the edges of the cover layer along the substrate act as capillary wicking paths that cause fluids to migrate along the edges of the cover layer. Such wicking is responsible for deleterious effect such as, for example but not limited to, corrosion and electrical shorting in systems such as inkjet printers where the flexible circuit is utilized to attach the inkjet print head to a printed circuit board.